1. Field of the Invention
The present invention relates to an illumination apparatus used in an image-taking apparatus or the like.
2. Description of Related Art
Conventional illumination apparatuses used in an image-taking apparatus such as a camera are made of a light source and an optical component, such as a reflector or a Fresnel lens, which guides light flux emitted from the light source to a frontward direction (toward the object).
Various such illumination apparatuses have been proposed, in which light emerging from the light source in various directions is condensed to the necessary illumination angle within little space and with high efficiency.
In particular in recent years, illumination apparatuses have been proposed in which, instead of arranging a Fresnel lens to the front (on the object side) of the light source, a higher condensing efficiency and greater compactness is achieved by arranging an optical member utilizing total reflection, such as a prism or a light guide.
For example, Japanese Patent Applications Laid-Open No. 2000-250102 and 2003-287792 propose illumination apparatuses having a light source and an optical prism for irradiating light flux emitted from the light source toward the irradiated side. In this apparatus, the optical prism has a first incident surface on which the light flux from the light source that emerges near an irradiation optical axis is incident, an emerging surface from which the light flux from the first incident surface emerges, a second incident surface on which some of the light flux from the light source emerging at a larger angle than the light flux near the irradiation optical axis is incident, and a reflection surface which slants the light flux from the second incident surface and lets it emerge from the emerging surface. These surfaces are configured to have such surface shapes that there is a constant correlation between the angle defined by irradiation optical axis and the light flux emitted from the light source center, and the emergence angle with respect to the irradiation optical axis after the light flux has passed through the emerging surface.
Here, in view of mold ability and cost, an optical resin material such as an acrylic resin is usually used as the material of the optical prism.
However, in this type of illumination apparatus, a large amount of heat develops when light is emitted from the light source. With regard to the influence of this heat, the optical material needs to be selected and a heat dissipation space needs to be set in consideration of the thermal energy for one emission of light and the minimal light emission period.
Consequently, in the illumination apparatus proposed in Japanese Patent Applications Laid-Open No. 2000-250102, a certain distance between the light source and the second incident surface needs to be ensured in the optical prism irradiating the light flux from the light source toward the irradiation side, and it was difficult to make the optical prism smaller in the height direction.